This proposal is to study an important aspect of control of the sympathetic nervous system, particularly as it may relate to cardiovascular control in mammals. We have developed a unique and powerful system for studying this problem that will allow the performance of studies difficult or impossible in other experimental systems. Sympathetic preganglionic neurons (SPNs) located in the intermediolateral (and intermediomedial) cell columns of the thoracic and lumbar spinal cord are the final common pathway for control of sympathetic activity. Their activity is a fundamental determinant of ganglionic activity, and ultimately of organ function, including the heart, vasculature, gut, sexual organs, etc. The goal of this project is to determine the factors that control SPN activity. Studies will be performed in a new and powerful in vitro experimental system, which consists of the brainstem and spinal cord. Under appropriate conditions, complex nervous system function is maintained, including generation of rhythmic respiratory activity and of rhythmic locomotor activity as well as activity in SPNs. Thus, we will perform our experiments under excellent conditions for pharmacological studies and with intact, functional neural networks. In particular, by altering the extracellular medium by addition of pharmacological agents affecting receptors, channels or transmitter action, or by ionic substitution while monitoring intracellular potentials under current- or voltage-clamp conditions, we will be able to obtain critical information concerning the intrinsic and synaptic properties controlling the behavior of SPNs. We will focus on the descending brainstem projections from the rostral ventrolateral medulla to SPNs, and will address the following questions. 1-What are the intrinsic properties of sympathetic preganglionic neurons that determine their response to synaptic inputs? 2-What neuromessengers are released at the presynaptic terminals of medullispinal neurons, including those of the rostroventrolateral medulla, onto spinal sympathetic preganglionic neurons? 3-What are the mechanisms of the pre- and post-synaptic action of these neuromessengers, as well as those of other possible transmitters or modulators acting at this synapse? Homeostasis in health, including cardiovascular homeostasis, depends critically on the transmission of sympathetic drive to SPNs, and it is important in the physiological responses to (cardiovascular) diseases of organ (cardiac or vascular) origin. Moreover, dysfunctions in central regulation of cardiovascular function may underlie such serious conditions as essential hypertension and stress-induced arrhythmias. Thus, understanding the neuropharmacology of this drive will be of considerable importance in defining the mechanisms responsible for cardiovascular homeostasis and for pathologies, such as neurogenic hypertension. Furthermore, the rational development of therapeutic drugs for cardiovascular problems that can be treated or ameliorated by manipulation of central autonomic activity affecting the heart and vasculature depends on such knowledge. In particular, if we can identify the neurotransmitter(s) transmitting sympathetic drive to SPNs, then drugs can be synthesized that can control their excitability. This could allow for precise control of tone of cardiac and vascular smooth muscle.